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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 3,451,149 $ 5,271,818
Gross Unrealized Gains 133,052 145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 38,195 108,043
Available-for-sale Securities, Current 3,546,006 5,309,161
Estimated Fair Value 3,546,006  
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 32 23,832
Below Investment Grade Securities and Non-Rated Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses 17,838 100,503
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value 660,063 1,855,694
Corporate Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value 2,737,286 2,914,145
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 430,405 557,476
Gross Unrealized Gains 23 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 97 209
Estimated Fair Value 430,331 557,270
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0 0
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 3,020,744 4,614,623
Gross Unrealized Gains 133,029 145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 38,098 107,834
Estimated Fair Value 3,115,675 4,652,172
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 32 23,832
Fixed Income Securities [Member] | Municipal Bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 882,631 845,778
Gross Unrealized Gains 14,364 3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 17,076 69,400
Estimated Fair Value 879,919 779,834
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 5 0
Fixed Income Securities [Member] | Corporate Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,288,882 858,774
Gross Unrealized Gains 6,444 6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 17,204 5,471
Estimated Fair Value 1,278,122 860,075
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0 0
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 30,496 26,245
Gross Unrealized Gains 399 409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 61 111
Estimated Fair Value 30,834 26,543
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0 0
Fixed Income Securities [Member] | U.S. Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 93,636 86,900
Gross Unrealized Gains 1,371 261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 613 1,753
Estimated Fair Value 94,394 85,408
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 221,825 2,214,512
Gross Unrealized Gains 37,575 67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 793 30,482
Estimated Fair Value 258,607 2,251,333
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 27 23,832
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 133,075 50,754
Gross Unrealized Gains 8 283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 1,727 0
Estimated Fair Value 131,356 51,037
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0 0
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 370,199 531,660
Gross Unrealized Gains 72,868 66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 624 617
Estimated Fair Value 442,443 597,942
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities 0 0
Fixed Income Investments And Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   5,172,099
Gross Unrealized Gains   145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments   108,043
Estimated Fair Value   5,209,442
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities   23,832
Collateral Pledged [Member] | Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 0 99,719
Gross Unrealized Gains   0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments   0
Estimated Fair Value $ 0 99,719
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities   0
Collateral Pledged [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   99,719
Gross Unrealized Gains   0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments   0
Estimated Fair Value   99,719
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities   0
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   $ 99,719